The key to understanding American politics these days is recognizing that it's easier to oppose a candidate or policy than it is to promote one of your own.

This has been the case, more often than not, for nearly 250 years. From the Boston Tea Party to Occupy, political leaders maximize success when they can demonize someone.

As a corollary, blank slates — candidates with little or no public-office history — often prove to be more effective agents for change than seasoned professionals who carry a lot of ideological or pragmatic baggage. Think Deval Patrick in 2006. Witness Barack Obama in 2008.

It seemed to me that these truths ran through the past week of political activity, from the Massachusetts Democratic State Convention in Springfield to Netroots Nation in Providence, with the Wisconsin recall election and presidential parrying in between.

Activists on both sides of the political divide are uniting mostly around their perceived enemies, rather than their own platforms and policies.

That's not too surprising — especially in times of trouble like these, when pretty much nobody is likely to have a track record of success.

Still, it seems to pose the question of what, in the end, anyone is for. Can purely oppositional activists actually expect anything from their new leaders — whether that might be Tom Barrett in Wisconsin, Elizabeth Warren in Massachusetts, or Mitt Romney in the White House?

QUEEN ELIZABETH

At its purest, you see these tendencies at work in Warren's US Senate campaign. Opposition to Kennedy-seat usurper Scott Brown is a given — and is deep and heartfelt. But so is the utter, unabashed enthusiasm for Warren, who secured the nearly unanimous backing of the state party at the Democratic state convention, and was the clear superstar at Netroots.

It's hard to put a finger on exactly why Warren is so much more compelling to liberal activists, in and outside the state, than other Democratic candidates — and, as I discovered, so hard for those activists to explain themselves.

Legendary environmental activist Bill McKibben told me he supports Warren enthusiastically, even though he conceded that she has said little, if anything, about environmental issues.

"Elizabeth Warren is in 2012 the marquee progressive that is fueled by everyday people, who are responding to her message," says Adam Green of Progressive Change Campaign Committee (PCCC) — which (like several other groups) began raising money for Warren last year, before she even became a candidate. Markos Moulitsas, founder of the Daily Kos Web site, told me that Warren is far and away the favorite candidate of the Netroots in this election cycle.

What Warren seems to have is a combination of résumé, symbolism, and rhetorical skill that makes her a great foil against the progressives' common enemy — which is not just Brown but, as McKibben suggests, the national Republican Party and the wealthy interests behind it.

Warren herself made that theme very clear in her convention speech — the first major public address of her political career, one could argue — by concentrating almost entirely on the evils of Brown. There was very little self-introduction, but plenty about the guy she is against. He is, in her words, "a Wall Street Republican; a big-oil Republican; a Mitt Romney Republican."

Brown Bagging If you are finding it hard to get enthused about the seemingly preordained drubbing that Democratic Attorney General Martha Coakley will give to the GOP nominee, State Senator Scott Brown, in the special election for US Senate, you are not alone.

Coakley for Senate When Massachusetts voters go to the polls on Tuesday to elect a successor to the late Senator Edward Kennedy, they face a choice that is as clear as the difference between black and white.

Tea-bagger Brown triumphs Massachusetts Attorney General Martha Coakley may be a good person and a dedicated public servant, but thanks to her gut-wrenching loss to tea-bagging Republican Scott Brown in the race for the US Senate seat held by the late Ted Kennedy, Coakley is now — quite rightly — a figure of local scorn and national derision.

How Brown won As the Massachusetts US Senate election unfolded yesterday, all that the pols and pundits wanted to talk about was how Martha Coakley managed to lose the race. And there is plenty there to dissect. But there is another part of the story, and that is how Scott Brown managed to win it.

What’s next for Cicilline? Providence Mayor David N. Cicilline rode into office seven years ago as the fresh-faced anti-Buddy. Bleach for a soiled City Hall.

Rhode Island’s own stab at health reform Health care reform, if it survives the election of Republican Scott Brown to Ted Kennedy's old Senate seat, will make insurance accessible and provide a slew of new consumer protections for millions of Americans.

Trying times for Obama It was only a matter of time before President Barack Obama turned into a deficit hawk. But it is a measure of the desperation sparked by Scott Brown's election to Ted Kennedy's old Senate seat that Obama hatched before the conclusion of the 2010 congressional elections and unveiled a spending freeze.

Elephant in the Room Platoons of state Republicans, energized by Scott Brown's stunning victory over Democrat Martha Coakley last week, are setting their sights on November.

MRS. WARREN GOES TO WASHINGTON | March 21, 2013 Elizabeth Warren was the only senator on the Health, Education, Labor, and Pensions (HELP) Committee, aside from the chair and ranking minority, to show up at last Thursday's hearing on indexing the minimum wage to inflation.

MARCH MADNESS | March 12, 2013 It's no surprise that the coming weekend's Saint Patrick's Day celebrations have become politically charged, given the extraordinary convergence of electoral events visiting South Boston.

LABOR'S LOVE LOST | March 08, 2013 Steve Lynch is winning back much of the union support that left him in 2009.

AFTER MARKEY, GET SET, GO | February 20, 2013 It's a matter of political decorum: when an officeholder is running for higher office, you wait until the election has been won before publicly coveting the resulting vacancy.